Body Fluid Compartments (B2: W5) Flashcards Preview

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Flashcards in Body Fluid Compartments (B2: W5) Deck (26)

What equation is used to estimate the volume of a body fluid compartment?

Volume = Quantity of tracer / Concentration of tracer

Divide that volume by body weight to figure out what percentage of body weight is water



What equation is used to find the intracellular fluid volume?

Intracellular fluid volume = Total water in body - Extracellular fluid volume


How do you find blood volume usine hematocrit? 

Blood volume = Plasma volume / (1 - hct)

Women hct: 0.38 - 0.46

Men hct: 0.42 - 0.54


Of the total body water (50-70% body weight), where is the distribution?

  • Greater volume inside of cells than outside: ~40%
  • Outside of cells: 
    • Interstitial
    • Plasma
    • Transcellular (joint fluid, CSF, etc.)


What assumptions do we make when considering disturbances of volume and osmolarity?

  • Na is essentially an extracellular ion
  • K is essentially an interacellular ion
  • The intracelluar compartment behaves as a perfect osmometer


What is the difference in osmolarity between the ECF and the ICF?


  • Osmolarity is the same
  • More volume in ICF
  • Also more partilces in ICF


What happens in the event of isotonic overhydration (careless over adminisration of saline)?

ECF compartment expands

  • Water stays put: isotonic 


What happens in the event of hypotonic overhydration (compulsive water drinking)?

  • Extracellular particles are diluted
    • Decrease in osomolarity
    • ECF compartment expands
  • Shifts water inside cells
    • ICF compartment expands 


What happens in the event of hypertonic overhydration (drinking sea water)?

  • There is an increase in extracellular osmolarity
    • Extra particles draw water outside of cells
  • ECF compartment expands
  • ICF compartment shrinks
    • leads to intense thirst


What happens in the event of isotonic dehydration (hemorrhage)?

  • Fluid loss contracts ECF compartment
  • Osmolarity remains the same: isotonic


What happens in the event of hypotonic dehydration (Addison's disease - Adrenal cortical insufficiency/lack of aldosterone)?

  • Drop salt from the body in excess in the urine
  • Osmolarity of the ECF goes down
    • ECF compartment contracts
  • Water goes towards more concentrated cells 
    • Expands ICF compartment 


What happens in the event of hypertonic dehydration (lost in desert/lack of ADH)?

  • Loosing water through sweating (more water than salt)
  • Particles become more concentrated
  • Water comes out of cells
    • ICF compartment contracts
  • Osmolarity goes up, blood volume goes down
    • ECF also shrinking due to sweat


What happens to sodium concentration in IV over administration? In hemorrhage?

Remains normal and constant with volume disturbance


What happens to sodium concentration with dehydration? Overhydration?

  • Sodium concentration can decrese with both
    • Water intoxication
    • Addison's disease (lack of aldosterone)
  • OR, [Na] can increases with both
    • Drinking sea water
    • Lost in the dessert


Since you can't distinguish if hypernatremia is a result of overhydration or dehydration, how can you tell them apart?

Look at plasma proteins

  • Decrease with drinking sea water (overhydration)
  • Increase with sweating (dehydration)


What is the difference in a large amount of sweat vs. a small amount?

Sweat is always hypotonic

  • With a small amount of sweat: more hypotonic
    • Time for reabsorption of Na from the tube
  • With a lot of sweat: less hypotonic
    • Approaches isotonic
    • More salt 


What happens to a healthy individual who loses 2 liters of sweat and simultaneously drinks 2 liters of pure water?

This person loses salt!

  • Plasma osmolarity will decrease
  • Intracellular osmolarity will decrease 
  • Exctracellular volume will decrease
  • Intracellular volume will increase 
  • Total solute content in the body will decrease 


Describe water turnover in a steady state

Water going in = water going out

  • Intake can be from water in food, metabolic water, or water in beverage
  • Water can leave in different ways
    • Insensible - lungs and skin
    • Sensible - urine, sweat, feces


How is plasma osmolarity regulated by ADH and thrist?

  • Osmoreceptors are in hypothalamus
  • Activation stimulates nerves
  • ADH released from posterior pituitary
    • Results in thirst
    • Regulates the kidney - water reabsorption 


What is the equation for osmolar clearance?

Cosm = (Uosm x V) / Posm

Total number of particles cleared in urine


How does osmolar clearance vary between isotonic urine, maximally dilute urine, and maximally concentrated urine?

Clearance cannot be determined by concentration

  • Each condition is capable of clearing the same volumeof plasma per minute
  • Consider flow


What is the equation for urine flow related to osmotic clearance and free water clearance?

V = Cosm + CH2O

  • Dilute urine: free water clearance is high
    • Urine is hypotonic
  • Concentrated urine: free water is reabsorbed
    • Urine is hypertonic
  • Isotonic urine: clearance of free water is 0
    • V = Cosm


Where is free water formed in the nephron?

Ascending limb of the loop of Henle

  • Salt moves out
  • Water stays in 
  • Hypotonic 


Where is free water reabsorbed back into the body?

Collecting tubule

  • Interstitial concentration reabsorbes water
  • Salt doesn't move much
  • Hypertonic urine


How do diuretics affect free water clearance?

  • We make free water in the ascending limb
  • If we stop making it, we are going to reduce it and clear less
  • Furosemide 


How do you determine if a diuretic is working in the ascending limb?

  • Look at the clearance of free water
    • The drug will create and clear less free water
  • Salty water is diuresed, but with particles 

Example: Drug Y